TRANSCRIPTIONAL AND TRANSLATIONAL REGULATION OF NITROGENASE IN LIGHT-DARK-GROWN AND CONTINUOUS-LIGHT-GROWN CULTURES OF THE UNICELLULAR CYANOBACTERIUM CYANOTHECE SP STRAIN ATCC 51142

Cyanothece sp. strain ATCC 51142 is a unicellular, diazotrophic cyanob acterium which demonstrated extensive metabolic periodicities of photo synthesis, respiration, and nitrogen fixation when grown under N-2-fix ing conditions. N-2 fixation and respiration peaked at 24-h intervals early in the dark or subjective-dark period, whereas photosynthesis wa s approximately 12 h out of phase and peaked toward the end of the lig ht or subjective-light phase. Gene regulation studies demonstrated tha t nitrogenase is carefully controlled at the transcriptional and postt ranslational levels. Indeed, Cyanothece sp. strain ATCC 51142 has deve loped an expensive mode of regulation, such that nitrogenase was synth esized and degraded each day. These patterns were seen when cells were grown under either light-dark or continuous-light conditions. Nitroge nase mRNA was synthesized from the nifHDK operon during the first 4 h of the dark period under light-dark conditions or during the first 6 h of the subjective-dark period when grown in continuous light. The nit rogenase NifH and NifDK subunits reached a maximum level at 4 to 10 h in the dark or subjective-dark periods and were shown by Western blott ing and electron microscopy immunocytochemistry to be thoroughly degra ded toward the end of the dark periods. An exception is the NifDK prot ein (MoFe-protein), which appeared not to be completely degraded under continuous-light conditions. We hypothesize that cellular O-2 levels were kept low by decreasing photosynthesis and by increasing respirati on in the early dark or subjective-dark periods to permit nitrogenase activity. The subsequent increase in O-2 levels resulted in nitrogenas e damage and eventual degradation.